• Bruce Metzger (NT professor at Princeton): “With the acquisition of î66 and î75, both of which read theos, the external support of this reading has been notably strengthened.” [A Textual Commentary on the Greek New Testament 2nd (Hendrickson, 2005), 198.]

This is relevant because an undisputed principle in textual criticism is Lectio difficilior potior (“the more difficult reading is the stronger”); in other words, the version which is more difficult to understand or to swallow is more likely the original insofar because it is more susceptible to having been “fixed” by a well-intentioned scribe.

For example:

“A majority of the Committee regarded the reading monogenes huios [only begotten son], which undoubtedly is easier than monogenes theos to be the result of a scribal assimilation to John 3:16.”[Ibid.]).

Taking “theos” as the original best explains why there was a change. As explained in Dallas Theological Seminary's esteemed

NET Bible Translator notes: “The external evidence thus strongly supports μονογενὴς θεός. ... As well, θεός also explains the origin of the other reading (υἱός), because it is difficult to see why a scribe who found υἱός in the text he was copying would alter it to θεός. Scribes would naturally change the wording to υἱός however, since μονογενὴς υἱός is a uniquely Johannine christological title (cf. John 3:16, 18; 1 John 4:9). But θεός as the older and more difficult reading is preferred.”

Similarly,

The Pillar New Testament Commentary: “No other passage puts these words together like this, which probably accounts for the change made by many copyists to monogenēs huios, ‘the unique and beloved Son’ (or, in more traditional language, ‘the only begotten Son’). That is so common an expression in John that it is hard to imagine any copyist changing ‘Son’ to ‘God’. Similarly, it is possible to explain the weakly-attested monogenēs, without either ‘Son’ or ‘God’ added, as an attempt to clear up the difficult reading with ‘God’ by simply dropping the latter; it is hard to imagine why any copyist would have added ‘God’ to monogenēs if this short form had been original.” [ed. Carson The Pillar New Testament Commentary (Wm. B. Eerdmans, 1991), 139.]

But so what?• While a majority of manuscripts do read “Son”, the fact remains that “a great mass of ancient evidence support the reading μονογενὴς θεός.”1• What is important is that “the best old Greek manuscripts (Aleph B C L) read monogenēs theos.” [See above]